The assumption that we ought to plan expressly for an incompatibility that essentially discards pg_upgrade seems premature, particularly in advance of would-be solutions that, in some cases, mightn't actually work.
If pg_upgrade doesn't work, then, at present, the plausible solutions are to either dump and restore, which might take way too long, or use one of the logical replication systems (e.g. - Slony, Londiste, or similar, in the absence of the would-be built-in logical replication).
Unfortunately, there are significant scenarios where none of these work, particularly for data warehouse-like systems where the database size is so large that the users cannot afford the disk space to construct a replica. It sure seems premature to intentionally leave that set of users out in the cold.